1. Field of the Invention
The present invention relates to the field of semiconductor device processing and, more particularly, to a gas feedthrough which provides processing gases into a processing chamber. More specifically, the present invention relates to a constant voltage gradient gas feedthrough made of a static-dissipative composite.
2. Background of the Related Art
In the fabrication of integrated circuits, chemical vapor deposition (CVD) is a well known process for depositing materials onto a substrate. CVD processes commonly require that process gases be delivered into a processing chamber where these gases undergo chemical reactions to form a desired layer on the surface of the substrate.
Gas distribution plates are used in CVD chambers to uniformly distribute the process gases delivered into the chamber. Uniform gas distribution is necessary to achieve uniform deposition characteristics on the surface of the substrate positioned in the chamber. In some CVD applications, an electrical current is applied to a gas outlet manifold on the gas distribution plate which introduces the process gases into the chamber to provide the necessary power to generate a plasma within the chamber.
Typically, an RF power source provides an energy potential to the outlet manifold which delivers the process gases into the chamber. The RF power generates a plasma within the chamber by exciting the process gases into a plasma state. Once excited, the precursor gases react within the chamber and the desired film typically deposits on the surface of the substrate.
In applications in which gases are introduced into the chamber through a gas distribution plate having an RF hot outlet manifold, energy applied to the outlet manifold must not be conducted to the gases being fed into the chamber. Typically, a grounded gas inlet manifold is provided upstream of the biased (i.e., RF hot) gas outlet manifold which provides the gases into the chamber and an insulative block is positioned therebetween to house gas lines which are typically made of quartz. A resistive sleeve is disposed around the gas lines to shield the voltage applied to the outlet manifold away from the gases fed through the gas lines while providing a resistive path between the inlet and outlet manifolds. The resistive sleeve is preferably held in place between the grounded gas inlet manifold and the RF hot gas outlet manifold by spring washers located at both ends of the sleeve.
In a typical system, a resistive sleeve of a composite material is positioned between the grounded inlet manifold and the RF hot outlet manifold to provide a path of resistance for the energy which is applied to the gas outlet manifold. However, the composite material currently being used requires that a metal contact be formed on the ends of the resistive sleeve to facilitate transfer of energy from the RF hot manifold to the resistive sleeve. The ends of the composite sleeves must undergo a metallization process to provide contacts which abut the inlet and outlet manifolds. It has been found that over time, the contacts formed on the ends of the composite sleeves by metallization wear out or break down under high power applications, thereby compromising the function of the resistive sleeve and altering the impedance of the CVD chamber. Further, the assembly steps required to mount the sleeves with the spring washers all too often result in out-of-specification resistance through the resistive sleeve.
If energy is conducted to the gas, the electrical configuration of the chamber may be altered, thereby adversely affecting the operation of the chamber. For example, if the energy applied to the gas outlet manifold is conducted to the gas, an unstable situation can result thus affecting the characteristic chamber impedance in an unpredictable manner.
U.S. Pat. No. 5,725,675, incorporated herein by reference, provides a resistive sleeve for use in a gas feedthrough which does not require formation of metal contacts on the ends of the feedthrough. This feedthrough is made of a material which has improved electromigration performance so that a constant voltage gradient can be maintained along the length of the feedthrough. This structure provides a constant voltage gradient across the feedthrough to the grounded manifold. This structure adequately solved the problems to which it was directed, but it includes a number of separate elements which require construction and assembly. Further, this structure includes components at the ends of the feedthrough which may alter the ohmic contacts at these ends and thereby alter the electrical characteristics of the feedthrough in unpredictable ways.
Thus, there also remains a need for a gas feedthrough that is formed as a unitary structure that easily couples inlet and outlet manifolds with predictable and reproducible impedance of the assembled feedthrough. Such a gas feedthrough should be formed of the minimum number of elements and should be made of readily available materials. Further, such a gas feedthrough should eliminate the troublesome resistive sleeve, thereby simplifying the assembly process, and simultaneously improving the quality of the entire semiconductor processing apparatus.